U.S. patent number 8,253,503 [Application Number 12/876,485] was granted by the patent office on 2012-08-28 for atomic oscillator and control method of atomic oscillator.
This patent grant is currently assigned to Seiko Epson Corporation. Invention is credited to Taku Aoyama, Koji Chindo.
United States Patent |
8,253,503 |
Aoyama , et al. |
August 28, 2012 |
Atomic oscillator and control method of atomic oscillator
Abstract
A method of controlling an atomic oscillator includes generating
a resonant light pair in response to a center frequency signal and
a sideband signal, and setting the sideband signal so that an
electromagnetically induced transparency (EIT) phenomenon does not
occur in a gas cell of the atomic oscillator. The method includes
applying the resonant light pair to the gas cell and detecting an
intensity level of light transmitted through the gas cell. While
the sideband signal is set so that the EIT phenomenon is not
occurring, the center frequency signal is varied until a minimum
value of the intensity level is identified. A first frequency is
calculated by subtracting a predetermined frequency offset from the
center frequency at which the intensity level was equal to the
minimum value. A center frequency of the resonant light pair is set
to the first frequency for operation of the atomic oscillator.
Inventors: |
Aoyama; Taku (Setagaya,
JP), Chindo; Koji (Kawasaki, JP) |
Assignee: |
Seiko Epson Corporation
(JP)
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Family
ID: |
43729912 |
Appl.
No.: |
12/876,485 |
Filed: |
September 7, 2010 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20110063037 A1 |
Mar 17, 2011 |
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Foreign Application Priority Data
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Sep 16, 2009 [JP] |
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2009-214134 |
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Current U.S.
Class: |
331/94.1;
331/3 |
Current CPC
Class: |
H03L
7/26 (20130101) |
Current International
Class: |
H03L
7/26 (20060101); H01S 1/06 (20060101) |
Field of
Search: |
;331/3,94.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kinkead; Arnold
Attorney, Agent or Firm: Harness, Dickey & Pierce,
P.L.C.
Claims
What is claimed is:
1. An atomic oscillator comprising: a gaseous alkali metal atom; a
light source to generate a resonant light for generating an
electromagnetically induced transparency phenomenon in the gaseous
alkali metal atom; a high frequency generation unit that supplies a
high frequency signal to the light source and generates a resonant
light pair; a center frequency variable unit that supplies a direct
current signal to the light source and varies a center frequency of
the resonant light pair; a light detection unit that detects
resonant light of the resonant light pair transmitted through the
gaseous alkali metal atom and outputs a detection signal
corresponding to intensity of the transmitted resonant light of the
transmitted resonant light pair; an absorption detection unit that
detects a minimum value of the detection signal when the frequency
of the resonant light is varied; and a signal processing unit that
controls supply or stop of the high frequency signal outputted from
the high frequency generation unit, wherein the signal processing
unit compares the minimum value detected by the absorption
detection unit with the detection signal in a state where output of
the high frequency signal is stopped, controls the center frequency
variable unit to cause the detection signal to become larger than
the minimum value by a specified value, and sets a center frequency
of the resonant light pair to be lower than the frequency of the
resonant light corresponding to the minimum value.
2. An atomic oscillator comprising: a gaseous alkali metal atom; a
light source to generate a resonant light for generating an
electromagnetically induced transparency phenomenon in the gaseous
alkali metal atom; a high frequency generation unit that supplies a
high frequency signal to the light source and generates a resonant
light pair; a center frequency variable unit that supplies a direct
current signal to the light source and varies a center frequency of
the resonant light pair; a light detection unit that detects
resonant light pair transmitted through the alkali metal atom and
outputs a detection signal corresponding to intensity of the
transmitted resonant light pair; an absorption detection unit that
detects a minimum value of the detection signal when the center
frequency of the resonant light pair is varied; and a signal
processing unit that sets a frequency of the high frequency signal
to a specified frequency at which the electromagnetically induced
transparency phenomenon does not occur, wherein the signal
processing unit compares the minimum value detected by the
absorption detection unit with the detection signal in a state
where the electromagnetically induced transparency phenomenon is
stopped, controls the center frequency variable unit to cause the
detection signal to become larger than the minimum value by a
specified value, and sets the center frequency of the resonant
light pair to be lower than the center frequency of the resonant
light pair corresponding to the minimum value.
3. The atomic oscillator according to claim 1, wherein the gaseous
alkali metal atom is cesium, and a center frequency of the resonant
light pair is set to be lower than the frequency of the resonant
light corresponding to the minimum value by 100 to 300 MHz.
4. A control method of an atomic oscillator using a light source to
generate a resonant light for generating an electromagnetically
induced transparency phenomenon in an gaseous alkali metal atom,
the control method comprising: supplying a high frequency signal to
the light source to generate a resonant light pair; supplying a
specified direct current signal to the light source to set a center
frequency of the resonant light pair; controlling supply or stop of
the high frequency signal; varying intensity of the direct current
signal in a state where the supply of the high frequency signal is
stopped and detecting intensity of the resonant light transmitted
through the gaseous alkali metal atom; detecting a minimum value of
the detected intensity of the resonant light; and varying, based on
the detected minimum value, the intensity of the direct current
signal in the state where the supply of the high frequency signal
is stopped, detecting the intensity of the resonant light
transmitted through the gaseous alkali metal atom, and setting the
intensity of the direct current signal to cause the intensity of
the resonant light to become larger than the minimum value by a
specified value.
5. A control method of an atomic oscillator using a light source to
generate a resonant light for generating an electromagnetically
induced transparency phenomenon in an gaseous alkali metal atom,
the control method comprising: supplying a high frequency signal to
the light source to generate a resonant light pair; supplying a
direct current signal to the light source to set a center frequency
of the resonant light pair; setting a frequency of the high
frequency signal to a specified frequency at which the
electromagnetically induced transparency phenomenon does not occur;
varying intensity of the direct current signal to store a minimum
value of the intensity of the resonant light pair transmitted
through the gaseous alkali metal atom; and varying, based on the
stored minimum value, intensity of the direct current signal,
detecting the intensity of the resonant light pair transmitted
through the gaseous alkali metal atom, and setting the intensity of
the direct current signal to cause a detection value of the
intensity of the resonant light pair to become larger than the
minimum value by a specified value.
6. The control method according to claim 4, wherein the gaseous
alkali metal atom is cesium, and the specified value is set to
cause the center frequency of the resonant light pair corresponding
to the set intensity of the direct current signal to become lower
than the frequency of the resonant light corresponding to the
minimum value by 100 MHz to 300 MHz.
7. A method of controlling an atomic oscillator including a gas
cell, the method comprising: generating a resonant light pair in
response to a center frequency signal and a sideband signal;
setting the sideband signal so that an electromagnetically induced
transparency (EIT) phenomenon does not occur in the gas cell;
applying the resonant light pair to the gas cell; detecting an
intensity level of light transmitted through the gas cell; while
the sideband signal is set so that the EIT phenomenon is not
occurring, varying the center frequency signal of the resonant
light pair until a minimum value of the intensity level is
identified; calculating a first frequency by subtracting a
predetermined frequency offset from the center frequency of the
resonant light pair at which the intensity level was equal to the
minimum value; and setting a center frequency of the resonant light
pair to the first frequency for operation of the atomic
oscillator.
8. A method of controlling an atomic oscillator including a gas
cell, the method comprising: generating a resonant light pair in
response to a center frequency signal and a sideband signal;
setting the sideband signal so that an electromagnetically induced
transparency (EIT) phenomenon does not occur in the gas cell;
applying the resonant light pair to the gas cell; detecting an
intensity level of light transmitted through the gas cell; while
the sideband signal is set so that the EIT phenomenon is not
occurring, varying the center frequency signal until a minimum
value of the intensity level is identified; calculating a first
intensity value by adding a predetermined offset to the minimum
value; while the sideband signal is set so that the EIT phenomenon
is not occurring, decreasing a center frequency of the resonant
light pair starting from a first frequency , until the intensity
level is equal to the first intensity value, wherein the first
frequency is the center frequency of the resonant light pair at
which the intensity level was equal to the minimum value; and
setting a center frequency of the resonant light pair to a second
frequency for operation of the atomic oscillator, wherein the
second frequency is the center frequency of the resonant light pair
at which the intensity level was equal to the first intensity
value.
Description
BACKGROUND
1. Technical Field
The present invention relates to an atomic oscillator, and
particularly to a technique to generate an optimum frequency of EIT
to maximize EIT intensity in an atomic oscillator of an EIT
system.
2. Related Art
An atomic oscillator based on an electromagnetically induced
transparency system (EIT system, also called a CPT system) is an
oscillator using a phenomenon (EIT phenomenon) in which when two
resonant lights different from each other in wavelength are
simultaneously irradiated to an alkali metal atom, the absorption
of the two resonant lights is stopped. Only one pair of two
resonant lights different in wavelength is prepared, and the
frequency is controlled so that the frequency difference
(wavelength difference) between the two simultaneously irradiated
resonant lights accurately coincides with an energy difference
.DELTA.E12 between the respective ground levels. An initial
operation at the time when the atomic oscillator in a stop state is
started will be described. When the power source of the atomic
oscillator is turned on, first, a wavelength of a light source is
swept to find the bottom of an absorption band of an objective
alkali metal atom. That is, the minimum value of a detection signal
is detected when the center frequency of a resonant light pair is
swept, that point is determined to be an excitation frequency
(excitation wavelength), and an EIT signal is obtained.
U.S. Pat. No. 6,265,945 (patent document 1) discloses a structure
and an operation method of a CPT system small atomic oscillator of
a sideband system in which a surface emitting laser (VCSEL) is used
as a light source.
However, it is experimentally confirmed that the excitation
frequency (excitation wavelength) at which the maximum EIT signal
intensity is obtained is not the bottom of the absorption band and
is shifted to a low frequency side from the bottom of the
absorption band (FIG. 4 shows a case where the alkali metal atom is
a cesium (Cs) atom, and the EIT signal intensity is maximum at the
frequency of point P. The bottom of the absorption band is near 500
(MHz) in the value of the horizontal axis of the drawing.
Incidentally, the reference (zero point) of the absolute value of
the horizontal axis is arbitrary.). Thus, in the related art atomic
oscillator, it can not be necessarily said that the maximum EIT
signal is detected, and the degradation of S/N is caused.
Besides, in the related art disclosed in the patent document 1, the
frequency condition of two light waves to maximize the CPT (EIT)
signal intensity is not prescribed.
SUMMARY
An advantage of some aspects of the invention is to provide an
atomic oscillator in which a resonant light pair is generated by
supplying a sideband component of a frequency slightly shifted so
as not to generate EIT, the center frequency is swept in this state
to detect the bottom of a detection signal, and when the bottom is
detected, the center frequency of the resonant light pair is
shifted to a low frequency (long wavelength) side, and the sideband
modulation wave is swept, so that the maximum EIT signal is
detected, the S/N is improved and the frequency can be
stabilized.
Another advantage of some aspects of the invention is to solve at
least a part of the problems mentioned above and the invention can
be implemented as the following embodiments or application
examples.
APPLICATION EXAMPLE 1
According to this application example of the invention, an atomic
oscillator includes a gaseous alkali metal atom, alight source to
generate a resonant light for generating an electromagnetically
induced transparency phenomenon (EIT phenomenon) in the gaseous
alkali metal atom, a high frequency generation unit that supplies a
high frequency signal to the light source and generates the
resonant light pair, a center frequency variable unit that supplies
a direct current signal to the light source and varies a center
frequency of the resonant light pair, a light detection unit that
detects the resonant light transmitted through the gaseous alkali
metal atom and outputs a detection signal corresponding to
intensity of the transmitted resonant light, an absorption
detection unit that detects a minimum value of the detection signal
when the frequency of the resonant light is varied, and a signal
processing unit that controls supply or stop of the high frequency
signal outputted from the high frequency generation unit. The
signal processing unit compares the minimum value detected by the
absorption detection unit with the detection signal in a state
where output of the high frequency signal is stopped, controls the
center frequency variable unit so that the detection signal becomes
larger than the minimum value by a specified value, and sets the
center frequency of the resonant light pair. The set center
frequency is set to be lower than the frequency of the resonant
light corresponding to the minimum value.
In the application example of the invention, the signal processing
unit outputs an output control signal to control the supply and
stop of the high frequency signal from the high frequency
generation unit, and a frequency control signal to sweep the
frequency of the high frequency generation unit. When the minimum
value (bottom) of the signal generated when the center frequency is
swept by the center frequency variable unit is detected, the center
frequency (wavelength) is shifted to the low frequency (long
wavelength) side, the high frequency generation unit modulates the
light source by the high frequency, and an EIT signal is detected
by sweeping the high frequency by the signal processing unit. By
this, the maximum EIT signal can be detected, S/N is improved, and
the frequency of the atomic oscillator can be stabilized.
APPLICATION EXAMPLE 2
According to this application example of the invention, an atomic
oscillator includes a gaseous alkali metal atom, alight source to
generate a resonant light for generating an electromagnetically
induced transparency phenomenon (EIT phenomenon) in the gaseous
alkali metal atom, a high frequency generation unit that supplies a
high frequency signal to the light source and generates a resonant
light pair, a center frequency variable unit that supplies a direct
current signal to the light source and varies a center frequency of
the resonant light pair, a light detection unit that detects the
resonant light pair transmitted through the gaseous alkali metal
atom and outputs a detection signal corresponding to intensity of
the transmitted resonant light pair, an absorption detection unit
that detects a minimum value of the detection signal when the
center frequency of the resonant light pair is varied, and a signal
processing unit that sets a frequency of the high frequency signal
to a specified frequency at which the electromagnetically induced
transparency phenomenon (EIT phenomenon) does not occur. The signal
processing unit compares the minimum value stored by the absorption
detection unit with the detection signal in a state where the
electromagnetically induced transparency phenomenon (EIT
phenomenon) is stopped, controls the center frequency variable unit
so that the detection signal becomes larger than the minimum value
by a specified value, and sets the center frequency of the resonant
light pair. The set center frequency is set to be lower than the
center frequency of the resonant light pair corresponding to the
minimum value.
In the application example of the invention, the high frequency
signal is supplied to the light source from the beginning. However,
the frequency of the high frequency signal is slightly shifted from
the frequency at which the EIT occurs. By this, when the minimum
value of the detection signal is detected by sweeping the center
frequency, the EIT phenomenon does not occur. When the bottom of
the detection signal is detected, the center frequency is shifted
to the low frequency (long wavelength) side, and the high frequency
is swept by the signal processing unit, so that the EIT signal is
detected. By this, the maximum EIT signal can be detected, S/N is
improved and the frequency can be stabilized.
APPLICATION EXAMPLE 3
The gaseous alkali metal atom may be cesium (Cs), and the set
center frequency of the resonant light pair may be set to be lower
than the frequency of the resonant light corresponding to the
minimum value by 100 to 300 MHz.
It is experimentally confirmed that the frequency at which the EIT
signal becomes highest is shifted to the lower frequency side from
the bottom of the detection signal. Thus, in the application
example of the invention, the range may be set to 100 to 300 MHz.
By this, since the relation between the drive current (direct
current signal) of the light source and the frequency (wavelength)
is well known, the frequency can be easily set by setting the drive
current.
APPLICATION EXAMPLE 4
According to this application example of the invention, a control
method of an atomic oscillator uses alight source to generate a
resonant light for generating an electromagnetically induced
transparency phenomenon (EIT phenomenon) in an gaseous alkali metal
atom. The control method includes supplying a high frequency signal
to the light source to generate a resonant light pair, supplying a
specified direct current signal to the light source to set a center
frequency of the resonant light pair, controlling supply or stop of
the high frequency signal, varying intensity of the direct current
signal in a state where the supply of the high frequency signal is
stopped and detecting intensity of the resonant light transmitted
through the gaseous alkali metal atom, detecting a minimum value of
the detected intensity of the resonant light, varying, based on the
detected minimum value, the intensity of the direct current signal
in a state where the supply of the high frequency signal is
stopped, detecting the intensity of the resonant light transmitted
through the alkali metal atom, and setting the intensity of the
direct current signal to cause the intensity of the resonant light
to become larger than the minimum value by a specified value.
In this application example of the invention, the same operation
and advantage as those of the application example 1 of the
invention are obtained.
APPLICATION EXAMPLE 5
According to this application example of the invention, a control
method of an atomic oscillator uses alight source to generate a
resonant light for generating an electromagnetically induced
transparency phenomenon (EIT phenomenon) in an gaseous alkali metal
atom. The control method includes supplying a high frequency signal
to the light source to generate a resonant light pair, supplying a
specified direct current signal to the light source to set a center
frequency of the resonant light pair, setting a frequency of the
high frequency signal to a specified frequency at which the
electromagnetically induced transparency phenomenon (EIT
phenomenon) does not occur, varying intensity of the direct current
signal to store a minimum value of the intensity of the resonant
light pair transmitted through the gaseous alkali metal atom,
varying, based on the stored minimum value, the intensity of the
direct current signal, detecting the intensity of the resonant
light pair transmitted through the gaseous alkali metal atom, and
setting the intensity of the direct current signal to cause a
detection value of the intensity of the resonant light pair to
become larger than the minimum value by a specified value.
In this application example of the invention, the same operation
and advantage as those of the application example 2 of the
invention are obtained.
APPLICATION EXAMPLE 6
The gaseous alkali metal atom may be cesium (Cs), and the specified
value may be set so that the center frequency of the resonant light
pair corresponding to the set intensity of the direct current
signal is lower than the frequency of the resonant light
corresponding to the minimum value by 100 to 300 MHz.
In this application example, the same operation and advantage as
those of the application example 3 are obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described with reference to the accompanying
drawings, wherein like numbers reference like elements.
FIG. 1 is a block diagram showing a functional structure of an
atomic oscillator of a first embodiment of the invention.
FIG. 2A is a flowchart for explaining an operation of the atomic
oscillator of the first embodiment of the invention, and FIG. 2B is
a flowchart for explaining an operation of an atomic oscillator of
a second embodiment of the invention.
FIG. 3A is a view showing an output signal from a light detection
unit when the center frequency of a light source is swept, FIG. 3B
is a view when the bottom of the output signal is detected, FIG. 3C
is a view when the center frequency is shifted to a long wavelength
side, and FIG. 3D is a view when a high frequency signal (sideband)
is swept for Cs and an EIT signal is detected.
FIG. 4 is a view showing a relation between the EIT signal
intensity and the center frequency for Cs.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
Hereinafter, embodiments of the invention will be described in
detail with reference to the drawings. However, components, kinds,
combinations, shapes, relative arrangements thereof and the like do
not limit the scope of the invention but are merely explanatory
examples unless there is specific description.
FIG. 1 is a block diagram showing a functional structure of an
atomic oscillator of a first or a second embodiment of the
invention. The atomic oscillator 100 includes a light source 1 to
generate a resonant light for generating an electromagnetically
induced transparency phenomenon (EIT phenomenon) in an alkali metal
atom, a cell 2 containing alkali metal (hereinafter referred to as
a gas cell) in which the amount of light absorption is changed by
the wavelength of light from the light source 1, a sideband
generation unit (high frequency generation unit) 5 that generates
the resonant light pair by supplying a high frequency signal to the
light source 1, a center wavelength variable unit (center frequency
variable unit) 4 that varies a center frequency of the resonant
light pair by supplying a direct current signal to the light source
1, a light detection unit 3 that detect the resonant light pair
transmitted through the alkali metal atom and outputs a detection
signal corresponding to the intensity of the transmitted resonant
light pair, an absorption detection unit 6 that detects the minimum
value (bottom) of the detection signal when the center frequency of
the resonant light pair is varied, a signal processing unit 8 that
controls supply or stop of the high frequency signal outputted from
the sideband generation unit 5, and an EIT detection unit 7 that
synchronously detects the output of the light detection unit 3 and
detects an EIT state. Incidentally, the signal processing unit 8
outputs a frequency control signal 9 to sweep the sideband
frequency of the sideband generation unit 5 and an output control
signal 10 to supply or stop the sideband generated from the
sideband generation unit 5.
In the state where the output of the sideband signal is stopped,
the signal processing unit 8 compares the minimum value of the
detection signal detected by the absorption detection unit 6 with
the detection signal, and controls the center frequency variable
unit 4 so that the detection signal becomes larger than the minimum
value by a specified value, and sets the center frequency of the
resonant light pair. The set center frequency is set to be lower
than the center frequency of the resonant light pair corresponding
to the minimum value.
Besides, in the state where the electromagnetically induced
transparency phenomenon (EIT phenomenon) is stopped, the signal
processing unit 8 compares the minimum value detected by the
absorption detection unit 6 with the detection signal, and controls
the center frequency variable unit 4 so that the detection signal
becomes larger than the minimum value by a specified value, and
sets the center frequency of the resonant light pair. The set
center frequency is set to be lower than the center frequency of
the resonant light pair corresponding to the minimum value.
That is, in this embodiment, the signal processing unit 8 outputs
the output control signal 10 to control the supply and stop of the
sideband from the sideband generation unit 5 and the frequency
control signal 9 to sweep the frequency of the sideband generation
unit 5. When the absorption detection unit 6 detects the minimum
value (bottom) of the signal generated when the center frequency
variable unit 4 sweeps the center frequency, the center frequency
is shifted to the long wavelength side, and the sideband generation
unit 5 modulates the light source 1 by the high frequency signal.
The high frequency is swept by the frequency control signal 9
outputted from the signal processing unit 8, so that the EIT signal
is detected. By this, the EIT signal of the maximum intensity can
be detected, S/N is improved, and the frequency of the atomic
oscillator can be stabilized.
FIG. 2A is a flowchart for explaining the operation of the atomic
oscillator of the first embodiment. First, the signal processing
unit 8 brings the output control signal 10 into an ON state, and
the sideband (high frequency signal) is inputted to the light
source 1. However, the frequency of the sideband (high frequency
signal) at this time is slightly shifted so that the EIT phenomenon
does not occur (S1). For example, when the alkali metal atom is Cs,
since .DELTA.E12 is 9.192 GHz (or 4.596 GHz of the half value
thereof) in terms of frequency, the frequency is shifted from these
values. Next, the center frequency variable unit 4 sweeps the
center frequency (S2). At this time, as shown in FIG. 3A, the light
detection unit 3 outputs the detection signal like a waveform 20.
Besides, as spectra, a spectrum 21 of the light source 1 and a
spectrum 22 of the sideband appear. At this time, since the
frequency of the sideband is slightly shifted so that the EIT
phenomenon does not occur, the EIT phenomenon does not occur, and
the bottom (minimum value) of the detection signal is detected (Y
at S3) (see FIG. 3B). Incidentally, it is determined that the
bottom of absorption occurs when such a condition is satisfied that
the bottom of absorption continues for a certain time or the change
becomes a certain level or less. Next, when the bottom of
absorption is detected, the center frequency is shifted to a long
wavelength side (S4) (see FIG. 3C). The amount of shift is about
200 MHz. The signal processing unit 8 sweeps the sideband by the
frequency control signal 9 while the sideband is added (S5).
Thereafter, shift is made to sweep control, and the EIT detection
unit 7 detects the EIT signal (see FIG. 3D).
That is, in this embodiment, the sideband (high frequency signal)
is supplied to the resonant light pair from the beginning. However,
the frequency of the sideband is slightly shifted from the
frequency at which the EIT occurs. By this, when the minimum value
of the detection signal is detected by sweeping the center
frequency, the EIT phenomenon does not occur. When the bottom
(minimum value) of the detection signal is detected, the center
frequency is shifted to the long wavelength side, the sideband
generation unit 5 modulates the light source 1 by the sideband, and
the sideband is swept by the signal processing unit 8 to detect the
EFI signal. By this, the EIT signal having the maximum intensity
can be detected, S/N is improved, and the frequency of the atomic
oscillator can be stabilized.
Besides, it is experimentally confirmed that the frequency at which
the EIT signal becomes highest is shifted to the low frequency side
from the bottom (minimum value) of the detection signal. In this
embodiment, the range is set to 100 to 300 MHz. By this, since the
relation between the drive current of the light source 1 and the
frequency (wavelength) is well known, the frequency can be easily
set by setting the drive current.
FIG. 2B is a flowchart for explaining the operation of the atomic
oscillator of the second embodiment. The same step is denoted by
the same reference number and is described. First, the signal
processing unit 8 brings the output control signal 10 into an OFF
state and prohibits the sideband (high frequency signal) from
entering the light source 1 (S10). Next, the center frequency
variable unit 4 sweeps the center frequency (S2). At this time, as
shown in FIG. 3A, the light detection unit 3 outputs the detection
signal like the waveform 20. Besides, as a spectrum, only the
spectrum 21 of the light source 1 appears. At this time, since the
sideband is not inputted, the EIT phenomenon does not occur, and
the bottom (minimum value) of the detection signal is detected (Y
at S3) (see FIG. 3B). Incidentally, it is determined that the
bottom of absorption occurs when such a condition is satisfied that
the bottom of absorption continues for a certain time or the change
becomes a certain level or less. Next, when the bottom of
absorption is detected, the center wavelength is shifted to the
long wavelength side (S4) (see FIG. 3C). The amount of shift is
about 200 MHz. Next, the signal processing unit 8 brings the output
control signal 10 into an ON state and inputs the sideband to the
light source 1 (S11). The signal processing unit 8 sweeps the
sideband by the frequency control signal 9 while the sideband is
added (S5). Thereafter, shift is made to sweep control, and the EIT
detection unit 7 detects the EIT signal (see FIG. 3D).
That is, in this embodiment, the sideband (high frequency signal)
is not supplied at first. By this, when the center frequency (the
frequency of a resonant light) is swept and the minimum value of
the detection signal is detected, the EIT phenomenon does not
occur. When the bottom of the detection signal is detected, the
center frequency is shifted to the long wavelength side, and the
sideband generation unit 5 supplies the sideband to the light
source 1. Next, the modulation is performed by the sideband, and
the sideband is swept by the signal processing unit 8 so that the
EIT signal is detected. By this, the EIT signal having the maximum
intensity can be detected, S/N is improved and the frequency of the
atomic oscillator can be stabilized.
The entire disclosure of Japanese Patent Application No.
2009-214134, filed Sep. 16, 2009 is expressly incorporated by
reference herein.
* * * * *